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I came across this sentence and I just wanted to confirm that "〜なさ" in this context is used a means to be assertive, more so than "〜ない".

文章 (Taken from a Peanuts book, this is Lucy speaking to Snoopy)
「ルーシー：あなたにちょっと提案があるんだけど...　あなたの人生をもっと意味深くする提案よ...　でも、興味なさそうね...」「スヌーピー：犬の人生に意味は要らないのさ...」

Where normally I am used to seeing a "興味ない" I see "興味なさ".

From what I know, the "さ" particle is used for assertion in sentence endings.

Thanks for the help.